94 The Life-History of Insects 



clearly discernible in the adult, and their appendages 

 develop into the six legs. A variable number of 

 the hinder abdominal segments become fused together 

 or suppressed in the adult insect, while in no insect, 

 after hatching, except certain Bristle-tails, are all the 

 abdominal limbs present, though those of the ninth 

 and eleventh segments (59) may persist as stylets and 

 cercopods respectively ; while in the immature stages 

 of moths and sawflies, as many as eight pairs may be 

 functional as " pro-legs." 



The segmentation of the embryo is of great interest 

 because it is believed to give some clue to the nature 

 of the remote ancestors of insects. We see that 

 every insect passes through a stage showing a number 

 of similar segments with similar rudimentary limbs, 

 some of which ultimately become transformed into 

 feelers, others into jaws, others into legs, and 

 others into cercopods. This fact suggests that from 

 ancestors with numerous similar segments, perhaps 

 distantly resembling centipedes, all insects have been 

 developed, the segments of the different regions of 

 the body and their limbs having become modified 

 in different ways for the performance of special 

 functions. 



Air-tubes. — The breathing-tubes, like the fore and 

 hind-gut, are formed by inpushings of the ectoderm 

 (fig. 62 J-). These arise in close association with the 

 developing appendages, the pits which mark the 

 future spiracles appearing shortly after the budding 

 limbs. As development proceeds, these pits lengthen 

 into tubes, which by repeated branching gradually 

 assume the complicated arrangement found in the 

 insect after hatching. 



Nervous System. — The nervous system of all 

 animals is developed from the outer skin or ectoderm 

 of the embryo. In insects a median groove is formed 



